The present invention relates to gas cooled generator stators and, more particularly, to a structure and method for impingement cooling of generator stator coils.
During the process of producing electricity, power generators also create heat that must be dissipated away from the generator. Generators are typically gas-cooled by ventilated cooling systems that circulate cooling gasesthrough ducts in the rotor and stator.
By way of example, FIG. 1 shows a cross-section of one-half of a generator 10, (see axial center line 12 and longitudinal center-line 14) havinga reverse flow ventilated cooling system. In this example, a portion 16 of the flow of cooling gases is directed to the rotor 18. The cooling gases are drawn through ventilation ducts 20 in the rotor by centrifugal forces created by the spinning rotor. As the gases flow through the rotor, heat in the rotor is transferred to the gases. Thus, heated gas exits the rotor ducts 20 at the surface of the rotor 18 and enters an air gap 22 between the rotor 18 and stator 24. Fans 26 mounted at the ends of the rotor 18 (only one of which is shown in FIG. 1) draw these heated gases through the annular gap 22 and direct the same via an external duct 28 to a heat exchanger 30 for cooling the gas.
The stator 24 is cooled by ventilation flow path(s) that are separate from the flow paths in the rotor 18. Gas 32 cooled by the heat exchanger 30 enters a plenum chamber 34 surrounding the stator 24. Cooling gas tends to flow in greater volume and velocity through stator ducts near the ends of the stator because the end sections of the stator are closest to the exhaust fans 26. This potential imbalance in the flow of cooling gas through the stator is preferably compensated for by varying the spacing and cross-sectional area of stator cooling ducts along the length of the stator to optimize the distribution of cooling gases through the stator and minimize the necessary pressure head needed for the cooling gases. The cooled gas flows to the stator outer circumferential surface 38 and into the cooling ducts 40 defined between the packets 42 of stator core laminations.
Referring to FIG. 2, the armature bars 44, 46 are secured in the stator coil slot 48 with filler 50, top-ripple spring(s) 52, and stator wedge 53 to restrain the bars radially, and with side-ripple spring(s) 54 to increase friction between the bars 44, 46 and the side walls of the slot 48. Referring to the lower armature bar 44 for convenience, the heat that is generated in the copper strands 56 of the armature bar 44 is thermally conducted along a heat flow path 58 from the strands 56, through a layer of insulation 60 to the side walls of the slot 48.
As noted above, cooling ducts 40 are incorporated into the stacks of laminations defining the stator core 24. Referring to FIG. 3, space blocks 62, 64 are provided axially between adjacent stator core packets 42 for defining the axial dimensions of the ducts 40 and for directing the cooling air flow radially through the stator 24. As the gas flows radially inwardly through the stator 24, heat from the stator coils 44, 46 is transferred to the gas. In conventional systems, the cooling gas ducts 40 are open ended so that the cooling gas flows radially directly into the annular gap 22 between the rotor 18 and stator 24 and then flows axially along that gap under the influence of the fans 26 for return to the heat exchanger 30.
As is apparent from the foregoing, the current-carrying copper conductors 56 of the typical stator coil/armature bar 44 are indirectly cooled. That is, in systems of the type described above, the coolant does not directly contact the current-carrying copper conductors 56 of the armature bar 44, nor indeed most of the bar 44. Instead, there is a thermal conduction path 58 from the armature bar 44 to the walls of the stator slot(s) 48. Thereafter the heat must be conducted through the lamination packet 42 to the adjacent cooling duct(s) 40.
This thermal conduction path 58, however, includes regions of imperfect contact between, e.g., the armature bar 44 and the side walls of the slot 48. The imperfect contact is inherent in the assembly of the multiple components. For example, because of the nature of the armature bar 44, it is not perfectly flat. Moreover, because of the assembly tolerance of the laminations that define the packets 42, the stack of laminations from which the core 24 is made may not align perfectly, so the slots 48 are not perfectly straight. More specifically, laminations may be slightly rotated clockwise or counterclockwise relative to a next adjacent lamination. Dead-air spaces are formed when individual adjacent laminations are slightly offset from each other in the peripheral direction. As a result, there are voids in the thermal conduction path, referred to as lamination stagger. Voids and imperfect contact of the type described above cause increased thermal resistance between the bar, which is the source of heat, and the cooling duct which is where the heat is taken away. High thermal resistance results in a higher operating temperature of the armature bar, which limits the output performance of the generator, since there is an imposed limit on the bar operating temperature.
In addition to the aforementioned imperfect thermal conduction paths in/along the stator slot, there is a further thermal resistance conduction path through the stack of stator core laminations. More specifically, once the heat is conducted through the thermal-contact resistance in the slot, heat needs to flow peripherally, radially and/or axially through the stack of iron laminations eventually to the cooling duct surface. The axial conduction path in particular presents a high resistance to heat flow since there is thermal contact resistance and often an enamel layer in between individual laminations in the stack.